Ohio officials have discovered 588 uncounted votes in this week’s special election for the 12th Congressional District, further tightening the already too-close-to-call race.

“The votes from a portion of one voting location had not been processed into the tabulation system,” the Franklin County Board of Elections said in a press release Wednesday. The uncounted votes were from a Columbus suburb.

Prior to the discovery, Republican Troy Balderson led Democrat Danny O’Connor by more than 1,700 votes. With the 588 votes included, the margin narrows to 1,564, according to the Cincinnati Enquirer.

Whatever happens in this special election, the two candidates will meet again in the November general election. But Republicans may be concerned that a district Trump won by a margin of 11 percentage points in 2016 has required a big mobilization of national resources.

The Franklin County Board of Elections will certify the final results on Aug. 24.